None of the factory 550's were delivered with soft covers and no snap or "tenax" fasteners were used. The spare holes from the full windshield were usally covered with a button head type bolt when not in use.
A lot of these cars were heavily modified and fixed over the years. There is a silver 550A (with the extra tubing for the frame) that is the most original 550 known to exist. It has never been modified, altered, etc. Play on the web and see if you can find it.
Heiko